1. Field of the Invention
The invention relates to corrosion-resistant cast iron and can be used in the metallurgy and foundry practice for the production of cast iron articles for use in chemical and petrochemical engineering.
It is common knowledge that segregation of main alloying components, which takes place during solidification of cast iron, results in a macroheterogeneous and microheterogeneous distribution of the alloying components. The segregation occurs even in the austenitic grain.
When such a cast iron is cooled down to a certain temperature depending on the macrodistribution and microdistribution of the alloying components the austenitic metal base partially dissociates to form martensite or bainite. The austenitic metal base dissociation causes a volume expansion of the cast iron, which in turn results in the increase of size of the resultant articles. This unwanted effect (formation of products of the austenite dissociation at the grain boundaries, and in the regions adjacent the graphite inclusions) affects to a great extent the corrosion resistance of the cast iron.
Therefore, apart from a high corrosion resistance, the corrosion-resistant cast iron should have a high resistance to its volume expansion. Hereinafter the term "expansion resistance" is used to mean a stability of the austenitic metal base, when the latter is subjected to a one-time or repeated cooling in the range of subzero temperatures, and the absence of phase transformations which lead to irreversible changes in the size of castings, and affect the corrosion resistance of the cast iron.
2. Prior Art
There is known a corrosion-resistant cast iron (USSR Author's Certificate No. 451,784) comprising, by mass %:
______________________________________ carbon 2.6 to 3.6 manganese 0.3 to 1.5 copper 0.5 to 9.0 magnesium 0.02 to 0.12 yttrium 0.01 to 0.10 tin 0.01 to 0.10 silicon 2.0 to 3.4 nickel 14 to 17 chromium 0.01 to 1.8 calcium 0.01 to 0.15 rare-earth metals 0.01 to 0.10 aluminium 0.005 to 0.3 iron balance ______________________________________
Apart from high physical and mechanical properties this cast iron features resistance to corrosion when exposed to such corrosive media as ammonia liquor, sodium hydroxide, trisodium phosphate, perhydrol, calcium hydroxide, and also methanol, benzene, and carbon tetrachloride.
The prior art cast iron, however, shows a low corrosion resistance in petroleum saturated with hydrogen sulfide, and in the water having an elevated content of cations of iodine and bromine (iodine-bromide water).
Furthermore, said cast iron shows expansion resistance only at a temperature higher than -45.degree. C., which limits its application in the chemical and petrochemical engineering.